Titanium and its alloys are finding increasing use in medical devices, including heart valves, cardiac pacemakers, bone plates, artificial joints and dental implants. There has been considerable interest, especially in the use of Ti-6Al-4V alloy, for orthopaedic implants because of its biocompatibility and fatigue strength. Occasionally, however, metal ions, particularly aluminum ions, have been found in tissue adjacent to titanium implants. Inflammatory and toxic effects associated with such metal release have also been reported. The problem of ion release is particularly of concern in the case of porous-coated implants. It should be noted that a prevailing method of achieving component stability of orthopaedic joint replacement devices is now by the use of porous-coated implants. High surface area, porous-coated implants have been shown to exhibit higher corrosion rates compared with conventional non-porous implants. There has also been some question of the wear resistance of Ti-6Al-4V against ultra high molecular polyethylene (UHMPE). For example, the wear rate has been observed to be about 100 times that encountered with the stainless steel or cast CO-Cr-Mo alloy under the same conditions of testing.
A number of surface modification techniques have been developed in the past to improve the corrosion performance and the wear resistance of Ti surgical implant alloys. These include plasma spraying TiO.sub.2, nitriding, ion implantation and special passivation techniques. The main disadvantage of ion implantation techniques is that the wear resistant layer or the corrosion resistant coating obtained by these methods are not sufficiently thick. As a result, the implant may lose its surface properties in the long term. For example, it has been shown that an ion-implanted layer in pure titanium, initially about 0.3 micrometer thick was worn to a remaining thickness of about 0.1 micrometer in the contact zone, at the completion of one million cycles wear test. Another problem with the ion-implantation technique is that the homogeneous ion implantation on complex substrates is difficult to realize.
Thick TiO.sub.2 coatings can be applied to titanium alloys by plasma spray technique but there are major problems associated with this method. The plasma spray deposition technique is a line of sight process which produces a non-uniform coating when applied to porous surfaces. Non-uniform coatings can create a local exposure of the metal and may provoke a local increase in metal ion release. Because of the high temperature involved, the technique has also the potential to alter the microstructure of the metal substrate and may weaken the implant material's resistance to fatigue. Plasma spray techniques are also very expensive as only about 15% of the relatively expensive TiO.sub.2 powder sprayed is actually deposited on the target. Titanium oxide coatings may also be formed on titanium and titanium alloys by anodization at relatively low temperatures. However, in most cases, conventional anodizing solutions described in the prior art do not have any dissolving power on the oxide layer formed during anodization. Therefore, the oxide layer thickness does not exceed more than 1 to 2 microns. This is, clearly, not a sufficient oxide thickness necessary for the long term performance of implants and surgical devices. Anodizing solutions, having fluoride and chloride containing ions with strong dissolving power on the oxide, have also been developed which allow the formation of relatively thick but porous oxide coatings. The porous oxide coatings have important industrial applications such as (a) eliminating the tendency of titanium towards seizure by retaining lubricants and (b) promoting the bonding between titanium and polymeric coatings such as adhesives and paints. However, there are major concerns associated with the application of these coatings to prosthetic devices for use in the human body. Scanning electron microscopy has shown that these coatings are, indeed, highly porous. The pores are formed by localized fluoride or chloride attack on the oxide as it is formed during anodization. As a result, toxic compounds containing fluoride, chloride, Ti, or a combination of these are formed which may be retained in the pores of the oxide during the anodizing process. These compounds may gradually leach out in the body and interfere with normal tissue growth near the implant. Certainly, the first requirement for any material to be placed in the body is that it should be biocompatible and not cause any adverse reactions.